1. Field of the Invention
The present invention relates to piezoelectric transformer inverters, and particularly, the invention relates to piezoelectric transformer inverters used for lighting liquid-crystal-backlight cold-cathode tubes.
2. Description of the Related Art
In general, liquid crystal displays use a light source for displaying, since liquid crystal does not emit light A cold-cathode tube is used as a light source for a backlight. For example, Japanese Unexamined Patent Publication No. 7-220888 describes a discharge-tube driving device formed by incorporating a piezoelectric transformer. In this device, a chopper circuit is disposed between an inverter circuit driving a cold-cathode tube used for a backlight and an input voltage terminal, and the duty of the chopper circuit is controlled in such a manner that the tube current of the cold-cathode tube is maintained constant, by which the luminance of the backlight is maintained constant.
In the conventional art described in the above publication, however, since the output of the chopper circuit is set to be a DC voltage, the chopper circuit is required to be a DC--DC converter. In order to use the chopper circuit as a DC--DC converter, it is necessary to dispose an inductor and a capacitor used for smoothing and rectification. This leads to the increase of the number of components and loss.
In addition, Japanese Unexamined Patent Publication No. 9-107684 provides the structure of an inverter circuit having a frequency dimming function controlling such that a tube-current value is a desired value by using the frequency-gain characteristics of a piezoelectric transformer and a chopper circuit including no smoothing/rectifying components disposed at a stage before the inverter circuit. This arrangement permits an average input voltage to the inverter circuit to be maintained constant. Although efficiency is reduced as the voltage inputted to the inverter circuit changes in the frequency-dimming system, when such a chopper circuit is disposed, a relatively high efficiency can be maintained in a wide input-voltage range. Furthermore, since no smoothing/rectifying circuit is disposed, there is an advantage in that there is no loss in the smoothing/rectifying circuit.
In the conventional art of the above publication, the frequency of an output of an oscillator used in the chopper circuit is divided to be used as a driving frequency of the inverter circuit. Therefore, in this system, a single oscillator is required to serve as both the oscillator of the chopper circuit and that of the inverter circuit. As a result, it is necessary to integrate an integrated circuit controlling the chopper circuit and an integrated circuit controlling the inverter circuit into a single integrated circuit.
However, when actually designed, there are an application in which the input-voltage range greatly changes and an application in which a constant input voltage is supplied. In addition, more freedom in designing can be obtained and component cost can be reduced, when a chopper circuit is independent from an inverter circuit. Furthermore, in this case, it is possible to reduce power consumption by using an inexpensive bipolar PWM IC with a high input withstand voltage for the chopper circuit and a CMOS-IC with a low withstand voltage (approximately 7V) and small power consumption for the inverter circuit.
Thus, in the conventional art, since it is necessary to integrate the chopper circuit and the inverter circuit into a single integrated circuit, there are limitations in terms of freedom in designing, cost, and performance.
Meanwhile, Japanese Patent Application No. 10-274751 provides a system driving a chopper circuit and an inverter circuit by individual integrated circuits. When two circuits are disposed separately as individual integrated circuits, the limitations to designing seen in the above Japanese Unexamined Patent Publication No. 9-107684 can be avoided. However, in the conventional art provided in Japanese Patent Application No. 10-274751, in the chopper circuit, beats are generated by a frequency of an integer multiple of the frequency of the inverter circuit and the frequency of the chopper circuit, and the oscillation of a piezoelectric transformer causes ripples at a low beat frequency.
In other words, since the piezoelectric transformer performs voltage conversion through a mechanical oscillation, parts of the mechanical oscillation leak onto peripheral structural members such as a printed circuit board. In this situation, when the oscillation of the piezoelectric transformer ripples at a low frequency, oscillations of the peripheral structural members ripple at the same beat frequency. Thus, a low-frequency component detected by non-linear factors of the structural members, for example, chattering generated at the joints of the structural members appears, which causes the beats.